1. Field of the Invention
The present invention relates to LC resonant circuits with narrow tolerance limits for the resonant frequency, and more particularly to such circuits useful as channel filters in carrier frequency technology.
2. Description of the Prior Art
Resonant circuits of this type are customarily adjusted to the desired resonant frequency by means of tuning cores. For tuning with the requisite precision, tuning cores of this type must be exactly adjusted to a few degrees of angle. When used in channel filters in carrier frequency technology, high demands are made on the adjustment precision of the LC resonant circuits.
The pass-band of such filters lies between 10 and 100 kHz; the band width is about .+-.2kHz; and the channel spacing is about 300 Hz. To satisfy these requirements, relatively large inductance and capacitance values (1-100 mH and 0.5-5 nF) are needed combined with high resonant circuit quality, such as close tolerances and stable component elements. The main technical problem in the production of filters of this type is the tuning of the resonant frequency, which must have a precision tuning of approximately 1% (pars pro mille). According to the prior art this tuning is carried out by altering the inductance value of the resonant circuit coil. For this purpose, an air gap in the magnetic coil core is altered by introducing a tuning core. An arrangement of this type results in expensive mechanics for achieving the necessary precision and stability. The tuning operation is time consuming and difficult to automate. In addition, with the prior art the temperature coefficient of the resonant circuit is also influenced as a result of the tuning of the inductance. This is not the case when tuning can be done by altering the capacitance. The use of rotary capacitors for this purpose is, however, much too expensive for most cases, particularly if large capacitance values are required.